1. Field of the Invention
This invention relates generally to electrical power generating systems for spacecraft and more particularly to solar cell panel systems.
2. Description of the Prior Art
The performance of practically all spacecraft is limited to a great degree by the amount of electrical power that is available during flight. The most efficient power source has been found to be the conversion of sunlight into an electric current by an array of photo-voltaic or solar cells. In early models, the exterior surface of a spacecraft was usually covered with solar cell array sections which were size-limited by the spacecraft dimensions and efficiency-limited by shadowing and the effects of angular solar incidence.
As the size of spacecraft increased, the volume available for power-consuming pay loads increased as the cube of the diameter (V=4/3 .pi.r.sup.3), while the surface area went up only as the square (A=4.pi.r.sup.2). Accordingly, electrical power availability from surface-mounted solar cell arrays become less and less for each unit of pay load volume as the size of the craft increased.
A solution to this problem of improving the solar cell array electrical output was found to increase the array area by the use of a folded or rolled-up array which is mechanically deployed or unfurled after the spacecraft was placed in its operational orbit or trajectory. This scheme has proved to be practical in the zero-g environment of space since such an extendable array can be designed with low weight and small power requirements. It is also desirable that the array be retractable for periods of power flight associated with course correction maneuvers, docking operations, and during the disposal of spacecraft waste to prevent solar cell contamination.
The solar cells in the above-described deployable arrays and also in fixed arrays are typically mounted in panel systems wherein two or three adjacent rows of a plurality of series-connected solar cells are connected in parallel. The series-parallel groups are then connected via appropriate bus bars to the spacecraft power utilization and battery charging systems. Nearly all of these solar cell panel systems use diodes connected in series between each solar cell group and a solar cell bus bar to prevent total panel failure in event of shorts developing in a single solar cell string in a group, and to prevent major power loss in case of partial shadowing of the panel. Also, these diodes help prevent battery discharge through the panel when it is in eclipse, and help prevent local heating effects due to shadows or cell output anomalies. Conventional diodes have been used for this "blocking" purpose and are generally cylindrical in shape and, on rigid flat solar cell panels and cylindrical arrays, the large diameter of the diodes compared to the thickness of the solar cell cover glass has been accommodated by installing the diodes in holes drilled in the substrates.
In fixed cylindrical arrays comprising the craft's exterior surface, the mounting of the diodes in holes presents no serious problem. However, in the more desirable-larger surface-rigid fixed and foldable panel arrays, these holes present a serious structural weakness since they are usually aligned in a row at the end of the solar cell groups adjacent the junction of the panels and the main body of the spacecraft where structural loads are concentrated.
On a flexible roll-up solar array, which typically may have a 2 mil substrate and a 13 to 14 mil thick solar cell/cover glass, the large diameter diodes present major design problems. One solution has been to mount the diodes in holes drilled in the drum or take-up roller of the roll-up array and run isolated bus bars from each panel group to the drum where they are connected to diodes mounted in a heatsink.
Blocking diodes are especially necessary in a roll-up array where part of the array is retracted (i.e., rolled up on the drum) and part of the array is illuminated. The retracted section will act as an electrical load to the illuminated part unless the retracted section is blocked off by diodes. If it is not blocked off, current leaks through the retracted, roll-up sections will cause a temperature rise therein and they will accept more current, which further raises their temperature -- permitting still further increase in leakage current. These phenomena could continue until cell damage occurs and/or the retracted sections act as a dead short to the rest of the current-producing panel. It can thus be seen that without the blocking diodes, the output of the entire panel might be lost.
As roll-up or thin solar panels become larger and more complex by the addition of panel regulators, for example, the prior art techniques of using conventional diodes become cumbersome, the number of isolated bus bars increase, and the area available for heatsinking of diodes is restricted. Accordingly, it should be evident that a technique which would provide the necessary diode protection while eliminating the need for heatsinks, allow maximum usage of diode isolation, and minimize the need for numerous long isolated bus bars would constitute a significant advancement of the art.